In my experience, the police will only accept a breath / blood sample proving alcohol over the legal limit before they prosecute. A few years ago, a boy racer wrote off my car ( I wasn't in it at the time) and was unable to stand up through alcohol. Even so, by the time he was breathalysed, he was below the limit - no insurance, so no point pursuing the case any further. the police will never act on heresay evidence - the offence may be dangerous driving or driving without due care, but without legal proof, drinking and driving will not be part of the equation.